Arefin's father, Abul Kashem Fazlul Haq, a noted intellectual and writer, said: "I rushed to his office at Aziz Market and broke the padlock, and I saw him lying upside down in a massive pool of blood. They slaughtered his neck. He is dead."

Jamaluddin Mir, a police officer, told Al Jazeera that at least seven other people were trapped inside the office when the attack was carried out.

"The criminals introduced themselves as customers who were buying books and entered the publishing house," he said, adding that an investigation to identify the attackers was under way.

Abbas Faiz, Bangladesh Researcher at Amnesty International, said: "The situation is becoming increasingly dangerous for those brave enough to speak their own minds. The latest heinous criminal attacks are a deliberate assault against freedom of expression in the country.

"Given the horrific pattern of violence, we have reason to believe many other lives are now at risk."

Earlier attacks

Earlier in the day, publisher Ahmed Rahim Tutul - who also published Roy - and two writers were shot and stabbed by three men in the office of the Shudhdhoswar publishing house.

Police officer Mir said the assailants then locked the wounded men inside the office before escaping.

"We had to break the lock to recover them," he said.

The two writers were identified by police as Ranadeep Basu and Tariq Rahim. All three of the victims were hospitalised, and Tutul was in critical condition, Mir said.

The attacks come amid fears about the rise of religious militancy in Bangladesh.

No one immediately claimed responsibility for the latest attacks.

At least four atheist bloggers have been murdered in the country this year while the Islamic State of Iraq and the Levant (ISIL) group has claimed responsibility for three other attacks.

The local Ansarullah Bangla Team group had claimed responsibility for the blogger killings and recently threatened to kill more bloggers.